My first and most important rule was that I could eat only at fast-food chains. (I would consume at least three meals a day.)

That means no health-food-obsessed fast casuals like Sweetgreen or Chipotle. It's all about chains best known for burgers, fries, and fried chicken 24/7. (The one exception: I could drink alcohol.)

My secondary rule: I must try and eat as healthy as possible at these fast-food restaurants. My definition of health was pretty general. My meals should to add up to less than the FDA's daily recommended limit in calories, fat, and sodium, while providing me with enough protein to fill me up.

The challenge would last for one work week, from Monday to Friday.

2/

I started off the week at a familiar breakfast destination: Dunkin' Donuts.

If you eat the flatbread while it's hot, it's actually tasty and pretty hearty. However, the longer you wait, the more disturbing the congealed egg white and cheese appear. I pair the flatbread with a coffee and head to work.

3/

Lunch at Wendy's revealed just how gross grilled chicken could get.

Kate Taylor

After finding a reasonably healthy breakfast at Dunkin' Donuts, I was optimistic about what Wendy's would serve up for lunch. I'm generally a fan of the chain, and the Asian Cashew Chicken Salad fit all my qualifications (again, high sodium but low calorie and otherwise healthy), so I ordered it with high hopes.

These hopes were not met.

First, let me say the salad was not without its merits. The fire-roasted edamame was delicious. The dressing was nice. The cashews were spicy. But the grilled chicken was irredeemable.

At first, I ignored the floppy texture of the chicken. However, the more of the meat I ate, the more alien it seemed. It wasn't the taste — it was the strange consistency that seems unique to some fast-food chicken used to top salads.

I didn't finish the dish.

4/

Dinner was nearly thwarted by yet another disturbing ingredient.

Kate Taylor

I gave chicken another chance with McDonald's Artisan Grilled Chicken Sandwich. Surrounded by the smell of fries, it was hard to persuade myself to order the sandwich, but I followed through.

The chicken was certainly better than Wendy's salad, but on my third bite, I encountered the mealiest tomato ever to make contact with my tongue.

I quickly removed the offending tomato. However, the experience ruined the entire sandwich — which was disappointing because the bun was truly fantastic.

5/

By the end of Day 1 of the experiment, I was feeling a bit hungry, but not defeated.

Kate Taylor

I ended up drinking a beer at an event I attended later in the night to get the taste of mealy tomato out of my mouth, going to bed semi-hungry — which makes sense, since I did not ingest nearly as many calories as is normal or recommended.

Day 1 total:

Calories: 1095 (not counting the beer)

Protein: 86 g

Sugar: 33 g

Fat: 34.5 g

Sodium: 2,650 mg

Price: $16.27

6/

I woke up ready to take on the world … or at least McDonald’s.

Kate Taylor

Unfazed by the mealy tomato, I hit up McDonald's for a yogurt parfait and cup of coffee.

The yogurt is good, but very sweet, which made sense after I checked out the nutritional information.

Still half asleep, I had picked based on calories (a mere 150), not noticing the yogurt had 23 grams of sugar. That's nearly half of the daily recommended limit!

7/

On Tuesday, I discovered what I now know to be the key to the fast-food diet: customization.

Kate Taylor

Taco Bell has advertised not-so-healthy offerings like the Quesalupa more intensely than nutritious choices, but it has three secret weapons.

One: the "Fresco" menu which replaces cheese, rice, and sour cream with pico de gallo. Two: a fully customizable online ordering feature. Three: a customizable nutrition calculator that lets you see which ingredients are contributing the calories, fat, and sodium to any meal.

I got the Burrito Supreme Fresco, with chicken, which was salty, cheap, and delicious. I spent the rest of the day raving about it to anyone who would listen.

8/

I realized a major advantage of this diet plan was its convenience.

Kate Taylor

I'm not great at sticking to most diet or workout regimes — something always seems to come up, whether it be working late or drinks with friends. However, fast food is inescapable.

So, when I covered an event debuting Burger King's new grilled dogs, instead of quitting the plan, I simply restrained myself and only ate about half of each hot dog. Then, after the event, it was easy enough to pick up a bland but inoffensive side salad from another Burger King location to fill me up.

9/

Another benefit: the price.

Kate Taylor

Skipping buying groceries and avoiding going out to nice dinners with friends, I hoped that this would be a relatively inexpensive week in terms of my how much I spent on food. By the end of day two, I'd spent $30.

Day 2 totals (approximated due to lack exact hot dog data):

Calories: 1,100

Protein: 28 g (not enough!)

Sugar: 42 g

Fat: 43 g

Sodium: 2,780 mg (too high, yet again)

Price: $13.90

10/

High on my Taco Bell success, I decided to recreate the experience for breakfast.

Kate Taylor

It worked. I got coffee and the AM Grilled Taco, with just cheese, eggs, and pico de gallo. It's simple and only 170 calories. Honestly, I will probably order this again at some point in the near future.

11/

Then, at lunchtime, disaster struck.

Kate Taylor

I thought Wednesday would be a good day to try out KFC's grilled chicken. Obviously, the "F" in "KFC" stands for "fried," but since the chicken chain has the option on its menu, I figured the grilled chicken was worth a shot.

The first problem presented itself as I ordered. I was unable to swap out my two-piece chicken combo's sides of mashed potato with gravy and a biscuit for the healthier corn on the cob and green beans. I ordered the green beans anyway, with plans to give the mashed potatoes to someone else, and took my order back to the office.

As I opened my box, I noticed a strange amount of grease leaking onto my desk.

"That's odd," I thought. "Grilled chicken should not be this greasy."

It was not grilled chicken.

Whether through human error or my particular KFC location not serving grilled chicken, I ended up with the fried chicken combo. Since the experiment was if I could eat healthy fast food, as opposed to simply eating healthy, I decided my only option was to consume it.

12/

I will admit the chicken was tasty — unlike the disturbingly mushy green beans, which went into the trash after one bite.

Kate Taylor

This lunch alone convinced me that the most difficult restaurant to get a healthy meal is KFC — even if you order one, you can't be sure you'll get it.

13/

Dinner presented a second struggle.

Kate Taylor

Since I was near New York City's sole Steak 'n Shake location for dinner, I decided to order one of the salads I had seen in my online stalking of the chain. Unfortunately, the cashier told me the Manhattan location did not serve any salads.

Searching the menu for something that wasn't fried or more than 500 calories, I decided to give up and get a diet soda.

14/

A blast from the past saved the day.

Kate Taylor

The Subway diet was once a major health trend, but I had forgotten how "fresh" the chain was once regarded. Subway may have fallen out of favor as a health destination, but it provided a simple, straight-forward turkey sub that was exactly what I needed for dinner.

Day 3 total:

Calories: 1,255

Protein: 59 g

Sugar: 28 g

Fat: 58.5 g

Sodium: 2,965 mg (yet again, the most problematic aspect of my meal)

Price: $18.26

15/

Still, Subway is no longer the king of nutritious fast food.

Kate Taylor

On Thursday morning, I decided to revisit the sandwich chain for breakfast. Seeking another simple meal, I ordered the Egg and Cheese Breakfast Flatbread.

Watching the employee warm up the little disk of egg white turned my stomach a little bit. The actual flatbread didn't help much — it had a slightly strange, sweet flavor that I eventually tried to mask with leftover Taco Bell hot sauce. Subway may have healthy options, but the taste just isn't something I'll ever crave.

After eating this, I began to suffer from symptoms of what I believe to be the biggest danger of a 100% fast-food diet. My ratio of sodium-to-calories was supremely out of whack, and my body wasn't loving it. I felt puffy and just generally out of it as I tried to get my work done.

16/

At lunch, I finally discovered that there was one chain that had everything I needed.

Kate Taylor

No single chain — except the delicious, but high-sodium, Taco Bell — had really wowed me at this point in the week. It wasn't hard to find low-calorie options on fast-food menus, but it was difficult to find nutritious food that I would adopt into my normal diet.

Finally, on Day 4, all that changed.

Chick-fil-A helped inspire my decision to adopt the fast-food diet, as the chicken chain has recently been promoting a number of healthy offerings.

Eating Chick-fil-A's grilled nuggets and the kale-based "Superfood Side" was the first time all week I enjoyed what I was eating, while also feeling like the meal was healthy and filling. The two were great alone or together. My only regret was that I didn't get a 12-piece of grilled nuggets, as the eight-piece was not quite enough to fill me up before dinner.

As a result, I ate my Chick-fil-A Asian Salad at 5 p.m. It was so incredible I had to double check the nutritional info, because nothing that tasted so could could actually be healthy.

17/

After working out that night, however, I began feeling a bit woozy.

Kate Taylor

While leaving the gym at around 9 p.m., I realized I had only eaten 970 calories in the entire day. Chick-fil-A recommends eating small meals every few hours, meaning my salad was probably not going to be enough to last me through the night.

So, I visited an old reliable: McDonald's. Thanks to all-day breakfast, I could order an Egg McMuffin, which is actually one of the chain's healthier options. The snack was exactly what I needed.

Total for Day 4:

Calories: 1,250

Protein: 90 g

Sugar: 28 g

Fat: 46 g

Sodium: 3,100 mg

Price: $25.07 (the delicious Chick-fil-A came at a price)

18/

On my last day of the fast-food diet, I wanted to draw on everything I had learned.

Kate Taylor

So, for breakfast, I revisited Chick-fil-A. There were lower-calorie options than the Chick-fil-A minis, plus a cup of fruit, but I couldn't resist the tiny chicken. Plus, it was a reasonably high-protein option.

The breakfast also helped me realize that Chick-fil-A has crafted a truly successful menu from a nutrition standpoint by taking a multi-dimensional approach to health.

The chicken chain not only has options that make customers feel healthy (hi, kale salad), but also healthier takes on classics, like the minis. Chick-fil-A's menu has a certain depth that most others, filled with subpar salads and a few grilled sandwiches, lack.

19/

Lunch highlighted the triumphs and shortcomings of Taco Bell.

Kate Taylor

The Cantina Power Bowl, which is basically Taco Bell's take on Chipotle's burrito bowl, is easy to customize to your nutritional desires. However, it is packed with sodium: 1,590 mg.

At this point I'm also bummed that Taco Bell doesn't have any healthy side options. Most of my meals this week haven't been quite filling enough — in part because most chains don't have healthy sides. KFC and Taco Bell are the biggest failures on this front, but Wendy's and Burger King's sad side salads aren't much better.

20/

I finished my week with a futuristic meal at a McDonald’s location with a Create Your Taste kiosk.

Kate Taylor

McDonald's new Create Your Taste platform was intended to help the chain compete with more health-conscious fast casuals. I figured my last meal of the week was a good time to see if it was up to the task.

My order was complex: Sirloin burger, mushrooms, shaved Parmesan, grilled tomatoes, pickles, and onions. Instead of a bun, I opted for a lettuce wrap.

Figuring out the nutritional info of the burger was harder to manage than I thought it would be, requiring visiting McDonald's Australian menu, Googling individual ingredient calorie counts, and much more math than I enjoy.

21/

The burger was… fine.

Kate Taylor

There was way too much lettuce, and the meat wasn't that great after a few bites.

Honestly, after this dish I was more than ready to be done with the fast-food diet once and for all. And, thankfully, in a few hours, I was.

Day 5 total:

Calories: 1,295

Protein: 84 grams

Sugar: 30 grams

Fat: 58 grams

Sodium: 3,430 mg

Price: $24.82

22/

While I’m happy the week is over, it wasn’t as terrible as other people seemed to expect.

Kate Taylor

Overall, I felt pretty good throughout the week. I didn't feel over-stuffed or ill. I didn't love most of my meals, but the majority were fine.

I also only spent $98.32 on food for the whole week. That's less than I would normally spend on groceries plus a few lunches and dinners out in New York City. Further, it's a tiny fraction of the cost of eating at trendy, "fresh" restaurants for breakfast, lunch, and dinner every day.

Compared to other diet plans, it's a minuscule sum — the Sakara Life diet, a favorite of Victoria's Secret models, costs $109 per day.

23/

However, I did realize that fast-food chains’ understanding of ‘health’ is a little skewed.

Business Insider

It wasn't difficult at all to cut calories while on the fast-food diet. In fact, if anything, it was too easy. I wasn't eating enough calories.

On Monday and Tuesday, when I couldn't go to the gym, that was fine. However, the rest of the week, I didn't quite have the energy to work out as hard as I wanted to.

The biggest reason for this was the lack of healthy, filling, and tasty sides at most fast-food chains. Most restaurants should take notes from Chick-fil-A on this front, adding some superfood sides to their own menus.

24/

At the same time, I consumed way too much salt.

The Dietary Guidelines of Americans recommends limiting sodium to less than 2,300 mg a day. I easily exceeded that every single day of my diet — even when when I wasn't even close to the daily limit in terms of calories, fat, or sugar.

There is a well-establish connection between sodium intake and high blood pressure, a risk factor for heart attacks and strokes. If people are eating a lot of fast food — especially if it is "healthy" fast food — heart issues may be a more immediate concern than obesity.

25/

I can’t say if I’m “healthier” after my week of eating nothing but fast food.

Kate Taylor

I don't have before-and-after photos, and forgot to weigh myself before undergoing the experiment.

However, the challenge did teach me how to navigate fast-food chains' menus. While calorie counts are often clearly displayed, I learned it is important to keep an eye on sugar and sodium to see what foods are actually healthy.

It also gave me a new perspective on dieting. After I plugged nutritional information into a spreadsheet this week, it became clear that the most important thing wasn't always picking the healthiest item on the menu. Instead, it was balancing different elements over the course of the day to get enough protein, vitamins, and energy.

I'm not planning on switching to the fast-food diet full time. However, you'd better believe I'll be ordering some of my new favorites from Chick-fil-A and Taco Bell without feeling even a little guilty for chowing down on fast food.

There are plenty of "healthy" fast-food options out there — you just have to take the time to find them.